Wide dimming range gas discharge lamp drive system

Abstract

A stable output, widely dimmable light source (20) utilizing a gas discharge lamp (14) including an external igniter (12) which is utilized to provide lamp ignition at approximately the same time as an AC potential source (22) provides power to the lamp (14) cathodes. A high voltage pulse is applied to the external igniter (12) causing ionization of the lamp gasses at the start of each time period during which the ballasted AC potential source (22) is applied to the lamp (14). Average light output level is controlled by a light output selector (36) which controls lamp on time and thus the average light output of the lamp (44). During operation lamp (14) is turned on several times per second, such as sixty-eight and the AC potential source (22) has a relatively high frequency, such as 45 KHz. When utilized for providing backlighting for a liquid crystal display, the average lamp light output can be controlled in response to ambient light to provide proper backlighting in conditions which vary from total darkness to bright sunlight. For LCD applications lamp (14) has a serpentine shape to provide increased and uniform backlighting.

I claim: 
     
       1. A wide dimming range drive system for driving a gas discharge lamp from an AC potential source comprising: a conductor mounted external but in proximity to the gas discharge lamp;   igniter means connected to said conductor for applying a short duration, high voltage pulse to said conductor causing ionization of the gases in the gas discharge lamp;   drive means for applying the AC potential to the gas discharge lamp causing light output; and,   control means for controlling times during which said drive means applies the AC potential to the gas discharge lamp for controlling average light output of the lamp.   
     
     
       2. A wide dimming range drive system as claimed in claim 1 wherein: the gas discharge lamp has a serpentine shape; and   said conductor is a thin strip disposed on the serpentine shaped gas discharge lamp and extending along most of its length.   
     
     
       3. A wide dimming range drive system as claimed in claim 1 wherein: the short duration high voltage pulse has a duration of 2 microseconds or less.   
     
     
       4. A wide dimming range drive system as claimed in claim 2 comprising: a high light reflectivity member disposed to support said lamp;   insulating means for electrically insulating said highly reflective member from said conductor and wherein,   the short duration high voltage igniter pulse has a rapid rate of rise.   
     
     
       5. Dimmable backlighting for a display panel comprising: a fluorescent lamp;   a power supply having a relatively high frequency AC output;   igniter means disposed external of but in proximity to substantially all of said fluorescent lamp for ionizing the gases in said fluorescent lamp when a high voltage pulse of a relatively short duration with respect to the frequency of the high frequency AC output is applied to said igniter means;   drive means for applying a short duration, high voltage pulse to said igniter means and for connecting said power supply to said fluorescent lamp while its gases are ionized to turn said fluorescent lamp on; and,   control means for controlling said drive means for varying the time said power supply is connected to said fluorescent lamp for varying the average light output of said fluorescent lamp.   
     
     
       6. Dimmable backlighting as claimed in claim 5 wherein said igniter means comprises a thin flat electically conducting strip attached to the outside surface of said fluorescent lamp along substantially its entire length. 
     
     
       7. A dimmable fluorescent lighting system comprising: a fluorescent lamp;   an external igniter disposed on said fluorescent lamp for causing ionization of lamp gases when the external igniter has a high voltage pulse applied thereto;   a relatively high frequency AC voltage source;   gate means for applying said AC voltage source to the fluorescent lamp; and,   control means for triggering said gate for selected time periods to control the average light output of said fluorescent lamp and for applying a short duration, high voltage pulse to said external igniter at the start of each selected time period to start said fluorescent lamp.   
     
     
       8. A dimmable fluorescent lighting system as claimed in claim 7 wherein: said external igniter is a narrow thin electrically conducting strip running along substantially the full length of said fluorescent lamp.   
     
     
       9. A method of controlling the average light output of a gas discharge lamp which can be excited from an AC voltage source comprising the steps of: (a) disposing an external igniter in close proximity to the gases within the gas discharge lamp;   (b) applying a short duration, high voltage pulse having a rapid rate of rise to the external igniter to ionize the gases in the gas discharge lamp when the lamp is to be turned on;   (c) applying to the gas discharge lamp while its gases are ionized the AC voltage source for selected periods of time which are a function of the desired average light output; and   (d) varying the length of the periods of time when the AC voltage source is applied to the gas discharge lamp to vary the average light output of the lamp.   
     
     
       10. A method of controlling the light output of a fluorescent lamp which periodically has an AC voltage applied thereto for variable time periods comprising the steps of: disposing an external igniter on substantially the entire length of the fluorescent lamp;   applying a short duration high voltage pulse having a rapid rate of rise to the external igniter to ionize the gases in the fluorescent lamp in conjunction with immediately thereafter applying the AC voltage to the fluorescent lamp causing the lamp to turn on; and   varying the time periods during which the AC voltage is applied to the gas discharge lamp to change the average light output of the lamp.   
     
     
       11. A dimmable fluorescent lighting system as claimed in claim 7 wherein the high voltage, short duration pulse is less than approximately 2 microseconds.